(A) Field of the Invention
The present invention is related to a band-pass filter circuit and multi-layer structure thereof to increase rejection of the band-pass filter, and the related method for increasing the rejection.
(B) Description of the Related Art
It is commonly known that critical roles in modern communications systems are played by a range of radio frequency (RF) passive devices, such as inductors, capacitors, resistors, antennas, or balanced-to-unbalanced transformers. These are essential devices in a wireless communication system. The RF band-pass filter is usually the most crucial device among the RF front-end circuits, and is used for selecting the signals in the required band and providing attenuation to noise sources outside the pass-band. In a communication system, the filter is typically placed after an antenna and before a low-noise amplifier. A superior RF band-pass filter must have low insertion loss in the pass-band and high attenuation outside the pass-band. In the past ten years, in view of rapid development of handheld mobile apparatuses and wireless networks, the RF band-pass filter has become rapidly commercialized and enjoys high industry demand. Therefore, according to recent trends of reduction in size and weight of electronic devices, RF band-pass filters with superior electrical performance and minimal size are in great demand in the market.
RF band-pass filters are important in wireless communication systems. Because noises and interferences are severe in wireless transmission channels, the system needs to rely on a band-pass filter placed at a front end to suppress the interferences of image and harmonic signals. Therefore, a crucial consideration of filter design is how to increase the rejection ability of the filter outside the pass-band.
U.S. Pat. No. 7,109,829 discloses a three-order band-pass filter, which uses multiple cross-coupling paths between resonators to generate transmission zeros outside the pass-band. The frequency response of the filter show one transmission zero at low skirt and two transmission zeros at high skirt. This system is concise, but the coupling mechanism between resonators is not easily controlled so that the precise positioning of transmission zeros is also difficult to control.